The present invention relates to a light modulator for modulating intermediate infrared-light. In particular, the present light modulator utilizes population inversion caused by a cooperation of both the current flow and the magnetic field for modulating light.
In optical communication systems, light modulators, as well as light sources, transmitting media, light detectors, etc. have become important elements. Light having a wavelength from 0.8 to 1.5 .mu.m can be modulated by means of internal modulation using light-emitting-diodes or laser diodes which have been developed so far. The term "modulation" is used herein in the broadest sense to include modulating any properties of light in general, not always limited to intensity modulation, phase modulation, frequency modulation, etc. The development of low-loss optical fibers has made the use of light in the wavelength range from 0.8 to 1.5 .mu.m for communication possible. The light having such a wavelength is, however, heavily attenuated in the atmosphere, so that near-infrared or visible light can not be employed as the carrier in optical communication systems using the atmosphere as the transmitting medium.
Infrared light of a wavelength from 8 to 14 .mu.m is hardly attenuated in the atmosphere. Therefore, much attentions have been paid to light having a wavelength from 8 to 14 .mu.m as a useful frequency band for wireless optical communication systems using the atmosphere as the transmitting medium. The CO.sub.2 laser is one of the light sources meeting such a requirement on the wavelength, which is also able to emit light of various wavelengths. The light having a wavelength of 10.6 .mu.m is particularly suitable for that purpose because of its ease in producing a high powered beam as well as the excellent quantum efficiency. The CO.sub.2 laser has been developed for other purposes, such as for machining and for medical treatments, and therefore, it is technically accessible. But, no matter how excellent a light source may be, it can not be used for optical communications without appropriate light modulators. Heretofore, suitable light modulators working at this wavelength range have not been available. Accordingly, light having a wavelength of 8-14 .mu.m could not be used in optical communications heretofore.
In optical communication systems, light modulation is an important means by which audio and other informations are "carried" by the light wave. The light wave, in effect, acts as a carrier wave. Modulation of light can be achieved either by external modulation using a modulator or by internal modulation which is effected by the operation of the light source itself However, it is difficult to modulate internally a CO.sub.2 laser itself or the like. Hence, some external modulators are needed.
Modulation may be achieved by appropriately changing the physical properties of the modulator itself. The modulation effected by electrical means is especially regarded as ideal, since it has an excellent in high-speed response.
As the conventional types of modulator , the devices using Kerr effect, Pockels effect, Faraday rotation and the like, have been known. The Kerr effect is a phenomenon wherein the refractive index of the modulating medium varies in proportion to the square of the applied electric field. The Kerr cell using the above Kerr effect has been used as a modulator. The Pockels effect is a phenomenon wherein the refractive index of a modulating medium varies in proportion to the applied electric field. The Faraday rotation is a phenomenon wherein the plane of polarization of light is rotated in proportion to the magnetic field applied to the direction of propagation of light. Actually, the light modulation is made by changing the strength of the magnetic field and by changing the current passing through the exciting coil Thus, although the Faraday modulator utilizes a magneto-optic phenomenon, the electric current is the real means for the modulation.
As described above, modulators using an electro-optic effect as well as a magneto-optic effect have already been realized. However, those modulators are merely used for the modulation of visible and near-infrared lights. Those modulators cannot be used for modulating light of longer wavelengths than 2.mu.m because there are no materials which are transparent enough at that wavelength range and also exhibit useful electro-optic or magneto-optic effects.
Regarding the infrared region of wavelengths from 2 to 22 .mu.m, a modulator using a pn-junction of Ge has been proposed so far. The free-carrier absorption in Ge is proportional to the number of free carriers. With the change in the forward voltage applied to the pn-junction, the number of free electrons injected from the pn-junction increases or decreases, so that the light absorption will correspondingly increase or decrease. This enables us to make an intensity modulation using the change in the light absorption. However, this method has a serious disadvantage due to its large absorption-loss. It has a further disadvantage in that the response speed is not fast. Undoubtedly, the absorption-modulation method involving Ge has found few practical utilities.
In view of this, it is easy to understand why the infrared-light modulators, effective at the infrared region of wavelengths longer than 2 .mu.m, have not previously existed. The disadvantage of absorption loss is especially apparent at the light modulation using a 10.6 .mu.m CO.sub.2 laser; the incident light is so powerful that such a modulator having high absorption-loss can not be used because of the possible damage and destruction due to the heating up.
J.times.H Force
Hereinafter, the term `J.times.H force` (also known as the Lorenz Force) will often be used, wherein J represents current density and H represents magnetic field. The interaction of J, H and light in a crystal is utilized in the present invention, and hence the term `J.times.H force` has a significant meaning representing the interaction.
To the best of our knowledge, no light modulators heretobefore have made use of the effect of the J.times.H force. Thus, the wording of `J.times.H force` does not only articulate briefly the operational principle of the present invention, but also expresses its novelty, and, therefore, the effect produced by the J.times.H force is called "magneto-electro-optic effect".
Magneto-electro-optic Effect
The term `electro-optic effect` already has a well-defined meaning in the art. However, the term `magneto-electro-optic effect` is a new term defined by the inventor as will be explained hereinafter.
In the Faraday effect, the directions of propagating light and the applied magnetic field are parallel to each other, and the plane of polarization of light is rotated through the propagation. This effect is obviously different from the magneto-electro-optic effect herein described.
The Voigt effect occurs when the direction of the propagating light is perpendicular to that of the applied magnetic field. In this case, the wave number of the "ordinary mode" (being defined below) becomes different from the wave number of the "extraordinary mode" (defined below), so that a phase difference results between the two modes. Hence, an intensity modulation could be achieved by using the effect together with a suitable polarizer, although the efficiency is very weak.
"Ordinary mode" refers to when the electric-field vector, E.sub.rf, of light is parallel to the magnetic field. In this mode, the vector is not directly affected by the applied magnetic field. There exists an "extraordinary mode" when the electric-field vector of light has the component perpendicular to the applied magnetic field. In this mode, the vector is directly affected by the applied magnetic field. Electrons and positive holes make cyclotron motions in a plane perpendicular to the magnetic field, so that the effective dielectric constant varies depending on the applied magnetic field.
Since a phase difference is obtained between the two modes in the application of magnetic field, it seems that the effect may be utilized as a light modulator. However, this is not true. Even though a strong magnetic field is applied, the phase difference is too small to be utilized as a practical modulator. There is a limit in the strength of the applied magnetic field for its practical use, no matter how effective it may be.
The Voight effect pertains to the interaction of light with magnetic field in the configuration, k.perp.H, where k is the wave number vector of light. The inventor came up with an idea, as set forth hereinafter, of passing a high current J through the materials perpendicularly to both the magnetic field and the light propagation direction, i.e., in the configuration, k//J.times.H, J.perp.k, and J.perp.H, in which k is the wave-number vector of light representing also the propagation direction.
The Hall effect is used in general for determination of the carrier concentrations in materials: the voltage, V, induced across the plane perpendicular to both J and H is measured by passing a current J perpendicularly to the magnetic field. It is evident that there exist no light waves in the Hall effect. In order to distinguish from the Voight effect, the present inventor refers to the phenomenon concerning the present invention as "magneto-electro-optic effect"; it refers to the phenomenon that the physical properties of the incident light is affected by the J.times.H force as propagating through electron-hole plasmas supported by semiconductors. In this case the J.times.H force plays an essential role, instead of the magnetic field H in the Voight effect.
However, on the usage of the above terminology we must pay some attention to avoid confusion, since the present inventor has already succeeded in realizing a light-emitting diode and light-amplifying, laser diodes in the same configuration of J.perp.H using the same material (InSb), and called the phenomenon concerning the invention "magneto-electric-photo effect". As set forth, in the same configuration of J.perp.H, the use of the same material for composing a light-amplifyer, a laser diode as well as a light-emitting diode has been known heretobefore, however, the principle for light modulation based on a population inversion due to the J.times.H force as proposed here has never been reported.
Infrared-Emitting (Laser) Diode
The material used by the inventor as the typical example is a single crystal of InSb known heretofore as an infrared material. Various types of laser diode have been produced or proposed. Light-emitting diodes and laser diodes can be made of semiconductors of Group III--Group V compounds such as GaAlAs, GaP, InP and the like. Each emits only the light of wavelengths from visible to the near-infrared region; any practical semiconductor lasers working at room temperature and capable of emitting infrared light of wavelengths longer than 2 .mu.m do not exist hereunto.
The inventor conceived that intrinsic InSb crystals might be used for composing a semiconductor diode (laser), through the excitation of electrons and positive holes by the action of J.times.H force without using any p-n junction. Japanese Patent No. 1359409 (Jan. 30, 1987) relates to such an InSb semiconductor diode (laser) disclosed by the present inventor. In this device, a strong magnetic field is applied to the single crystal, and a current is passed perpendicularly to the magnetic field. Then electrons and positive holes are driven to the direction perpendicular to both the magnetic field H and the current J. Assume the direction of the drift to be the y-coordinate axis, then the population of electrons and positive holes will increase near the one surface perpendicular to the y-coordinate axis and decreases near the opposite surface. In other words, the population of electrons and positive holes is not uniform but increases along the y-coordinate axis.
As the number of electron-hole pairs increases, the recombination takes place accompanied with light emission. If the recombination radiation takes-place in-phase at the limit of high excitation, we may have a laser diode. The emission becomes either spontaneous or stimulated corresponding to the extent of the excitation. Anyway, the device can function as a light-emitting diode. In said Japanese Patent No. 1359409, the device has been named "magneto-infrared-emitting diode". However, the naming may be somewhat incorrect, because in this device a bulky InSb single crystal itself is used, but no pn-junction is used.
The population of electrons and positive holes is increased deviating from the thermal equilibrium values by applying the J.times.H force to the InSb single crystal. The pumping energy comes from the power supply, but the electric current has quite a different role from the current passing through the pn-junctions of the conventional light-emitting diodes.
In the case of gas lasers or solid lasers, the laser action is caused by the inter-level transitions of electrons. The population in an upper level is increased by some excitation, and the population in a lower level is decreased, resulting in `population inversion`. Here, each "level" means the energy-levels of electrons in isolated atoms.
In the case of semiconductor lasers, the interlevel transition of electrons is not utilized but the interband recombination of electrons and positive holes is utilized. A current is passed through the pn-junction in the forward direction thereby injecting minority carriers so as to bring radiative recombination of excess electrons and positive holes. Although this state is rarely referred to as "population inversion", but in a sense it can be regarded as a substantial "population inversion".
It may be considered that the said magneto-infrared-emitting diode realizes a population inversion in a bulky InSb single crystal through the action of the J.times.H force. The narrow band-gap of InSb (0.23 eV) is appropriate for obtaining an infrared emission of wavelength.about.5.3 .mu.m, and we can compose an infrared-light-emitting device using it.
An injection-type InSb laser diode in which the excitation (injection) is made by current passing through the pn-junction has already been proposed. However, because of the narrow band-gap such an injection-type InSb laser does not work unless the device is cooled down to very low temperatures. At the earlier stage, all laser diodes were operated only intermittently at low temperatures, and had no practical utilities until the continuous lasing at room temperature became feasible in 1971.
Even though a monochromatic light having a wavelength longer than 2 .mu.m could be obtained by using an InSb pn-junction type diode, the practical utilities must be limited by the operational temperature as low as 4.2 K. However, the present inventor has demonstrated that the pumping by use of J.times.H force is useful for realizing a laser diode, using a bulky InSb crystal, which functions at higher temperatures.
Infrared-Light-Amplifier
After realizing an infrared-light-emitting device by applying the J.times.H force to an InSb single-crystal, the present inventor attempted to compose a light amplifier using the same material.
Unequilibrium population of electrons and positive holes along the direction perpendicular to both J and H, is caused in the InSb single-crystal by the J.times.H force. The number of electrons and positive holes in the vicinity of one surface is increased so much, and ultimately the number thereof reaches as hundreds of times as the value in the thermal equilibrium state, when highly excited, resulting in a population inversion.
By passing through the inverted population region, the incident light, having a wavelength corresponding to the band gap, can be amplified by means of a successive stimulated emission: when the inverted population is realized the population of electrons and holes becomes highly degenerate, so that the absorption of the incident light occurs no longer and only the stimulated emission takes place, thus resulting in the multiplication of photons concerned. In the strict sense the energy of the incident photons, h.omega., must satisfy the relation as given by eq. (44) hereinafter in order to be amplified.
The allowed energy-width for the incident photons is so narrow, and hence the materials composing the active medium in the light amplifier should be selected suitably according to the wavelength of the incident light: the tunability for the wavelength expected from varying the strength of the magnetic field will be at most 10% in the field strengths up to 10 Tesla.
J.times.H Force Semiconductor Device
Hereunto the principles of semiconductor laser and light amplifier using the J.times.H force have been described. Although InSb crystal has been employed as an example, any semiconductors can be employed so long as the band-gaps are narrow enough to fit with the infrared region. For example, materials which can be employed are HgCdTe, PbSnTe, PbSnSe, BiSb, InAs and the like. Except for InAs all other materials are mixed crystals and the band-gaps can be shifted by changing the mixture ratio.
The present inventor has proposed composing semiconductor lasers by use of these materials. Since the principle does not involve the use of a pn-junction, it is not needed necessary to equip any pn-junctions. Thus, these semiconductor lasers can be composed even from semiconductors for which fabricating the pn-junctions is difficult. Furthermore, the band-gap, E.sub.g, can be changed by selecting the material. In other words, infrared semiconductor lasers having a wide range of wavelengths can be obtained.
This type of semiconductor lasers are loser is pumped by the J.times.H force, so that the internal modulation can be made by changing J or H. Therefore, the intensity modulation of the output of the semiconductor laser is easily achieved by modulating J. However, whatever materials are selected, all values of the band-gap energy are not always obtained. Consequently, all light can not always be freely modulated, even if the wavelength is limited to a region from 2 to 20 .mu.m.
As set forth, the present inventor has proposed a light amplifier utilizing the J.times.H force. Since all amplifiers could be regarded as a kind of intensity modulator, it may also be used as an intensity modulator. In this case, however, the wavelength available for the modulation is limited to a wavelength being amplified in the material, which is roughly given in the following equation (1). Thus, the utility as an intensity modulator is seriously constrained.
As is well known, the central wavelength, .lambda..sub.o, of the emission generated by the interband recombination in the material having a band-gap energy, E.sub.g, is given by ##EQU1##
Thus, the wavelength to be amplified or modulated by the inverted population is roughly determined by equation (1) for given band-gap energy, E.sub.g, of the materials concerned.
For convenience in practical use, the construction of an infrared-light modulator working at wider range of the wavelengths and at room temperature is strongly required. For example, if CO.sub.2 laser light could be modulated at high speed, the various practical applications must be developed.